Modern wireless communications systems such as the widely adopted GSM (Global System for mobile communications) and the UMTS (Universal Mobile Telecommunications System), which has been selected as the European third generation mobile communications standard, are capable of transferring various types of data over the air interface between the network elements such as a base station and a mobile terminal. For that purpose, both the GSM and UMTS utilize a mature duplex transmission method called FDD (Frequency Division Duplex) in which uplink and downlink transfer directions are realized on two different frequency bands (˜paired bands). FDD thus employs frequency domain separation of transfer directions and enables continuous two-way transmission. In addition to the FDD assigned typically for macro- and microcells the UMTS specification includes also a mode supporting more recent TDD (Time Division Duplex) technology for time domain separation of transfer directions to be used with unpaired frequency bands and providing mainly local area access in reference to picocells etc with a higher user density. In the UMTS frequency bands 1920-1980 MHz (uplink) and 2110-2170 MHz (downlink) have been paired for FDD operation meanwhile frequency ranges 1900-1920 MHz and 2010-2025 MHz are targeted to TDD operation.
The FDD and TDD concepts are further clarified with the help of FIG. 1 wherein a wireless communications device such as a mobile terminal or a communication enabled PDA (Personal Digital Assistant) 102 is connected to mobile network 118 that may be internally divided to a radio access network and a core network as in UMTS. A network element communicating directly with wireless communications device 102 is in this example called a base station 104 that forwards, e.g. in the case of GPRS (General Packet Radio Service), the data received from SGSN (Serving GPRS support Node) 106 and originally delivered by GGSN (Gateway GPRS Support Node) 108 finally to wireless communications device 102 over the air interface by exploiting the active DL (downlink) connection. Accordingly, wireless communications device 102 may send data to network side 118 by utilizing an established UL (uplink) connection.
The originated UL/DL data transfer may be realized through both FDD and TDD techniques. Should the FDD be selected, UL 112 and DL 110 transfer directions are separated in relation to their carrier frequencies. Thus these two bands having either equal or differing bandwidths with necessary separation called a guard-band are used for duplex data transfer. Paired bands solution is ideal for symmetric traffic like voice communications and video conferencing but as a downside, truly flexible and dynamic bandwidth allocation between UL and DL transfer resources is either impossible or relatively complex to implement.
For supporting multiple access either in a FDD or TDD based network, for example, TDMA (Time Division Multiple Access) technique as in the GSM system or (W)CDMA ((Wideband) Code Division Multiple Access) technique as in the UMTS system, or even both simultaneously (e.g. UMTS TDD), may be used. Furthermore, FDMA (Frequency Division Multiple Access) technique can be applied whenever a plurality of carriers exists for a transfer direction.
TDD approach supports relative altering of UL/DL capacity and that way asymmetrical traffic more easily than FDD as the UL and DL directions share the same frequency and the required separation occurs in temporal domain by allocating a single carrier for two distinctive sets of time slots, one for each direction. Time slots may be dynamically allocated in an identical manner for symmetric traffic 114, or alternatively, in an unbalanced manner 116 for e.g. typical Internet traffic (Web surfing application: heavy downlink traffic, almost non-existent uplink control data) in which case either the UL or DL direction may dominate over the other one as to the time use. Frequency resources are thus not pointlessly reserved for a passive transfer direction. As a drawback, TDD implies discontinuous transmission for both ends of a connection, and a risk of interference introduced between the transfer directions arises due to possibly overlapping UL/DL transmission. Guard period 118 (illustrated in the figure for a single UL time slot only for clarification purposes) is typically used in the end of each slot to avoid overlaps.
Publication WO99/38343 discloses an arrangement supporting both time and frequency division duplex technologies to enhance spectrum usage in multi-cell environments. Two base stations located in neighbouring cells but with a geographical separation may use the same frequencies in such a manner that the first base station having a connection to a first mobile station transmits at a certain time instant by utilizing a first frequency while the second base station having a connection to a second base station transmits (or receives) by utilizing a second frequency. Next, the first base station receives data on the second frequency while the second base station receives (or transmits) data on the first frequency. Then the above cycle restarts.
Notwithstanding the various existing data transfer arrangements that may even utilize different types of data connections and duplexing methods to some extent, situations still occur whereto none of the prior art methods seems to fit particularly well. On the other hand, neither TDD nor FDD provide the system with pure benefits as presented hereinbefore.